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Capacitive power transfer: how would you approach this?

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Ash Small

Thu Jun 28 2012, 12:15PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'll try and build something to test it. the keyword here is 'electrostatics'.

Static electricity doesn't require a return path. If there was a return path when you hold a VDG your hair wouldn't stand on end, surely?

EDIT: I'll start another thread if you prefer, if you consider that my comments are 'off topic' in this one.

Steve Conner

Thu Jun 28 2012, 12:21PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That is true, but static electricity can't transfer power. Power is voltage times current, and if the electricity is static there's no current.

Ash Small

Thu Jun 28 2012, 12:28PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

That is true, but static electricity can't transfer power. Power is voltage times current, and if the electricity is static there's no current.

but there is 'movement of charge', there is a 'potential gradient', and a potential gradient can result in a movement of electrons from one place to another. I'll try to devise something to illustrate/demonstrate my point.

Steve Conner

Thu Jun 28 2012, 12:59PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Movement of charge is current. If charges are moving around, it's electrodynamics, not electrostatics. That means you need to consider the displacement current.

In the case of your original circuit, you need to draw a capacitor from one of the top corner nodes back to ground. The circuit won't work without this.

Ash Small

Thu Jun 28 2012, 02:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm working on a test setup using my sig.gen. I have to go out later, but I'll post results as soon as I have them. I'm not sure if the sig. gen. will provide sufficient output without amplification, but I'll try this with and without the additional cap. that you suggest.

Pinky's Brain

Thu Jun 28 2012, 05:33PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

Ash Small wrote ...

Static electricity doesn't require a return path. If there was a return path when you hold a VDG your hair wouldn't stand on end, surely?

Strictly speaking it's only an electrostatic effect after the fact ... the moment you touch the VDG electrons flow into you and your hair, the amount of charge which flows into you can be determined by measuring the capacitance of your body (Q=C*V).

I might have made things unnecessarily confusing by erroneously saying the return path is to ground ... positing a capacitance to ground is just a convenient model, there are actually multiple mechanisms acting as capacitance ... some of which are in effect even when the ground is infinitely far away. Basically any conducting body will act as having a stray capacitance to ground and because it acts like that the Murata method works.

Avalanche

Thu Jun 28 2012, 05:36PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I've been reading some old books recently (ahem) and one talks about charging a metal body using electrostatic induction - ie. non contact.

If a metallic charged sphere (say positive) approaches the end of a metallic non-charged rod, the electrons in the rod will re-arrange themselves so that there is a charge gradient along the rod... (is this re-arrangement within the rod considered a displacement current?) you can then bleed off charge at the non-charge-bound end, but the charge at the other end is bound by the sphere so it stays, resulting in a negatively charged rod when you take away the sphere. If you then threw this rod 50 yards, you'd have an electric current, but I'm not sure what the return path would be...

I've actually forgotten what my point was, sorry! Maybe you could catch the rod and then discharge it, obtaining some power?

Edit - as far as the VDG goes, there's a current the way I understand it, due to ions. Your hair (or breakout point on your head) is ionising the air near it, the molecules of which become charged to the same polarity and are repelled (creating a wind), so to keep the electrons going there must be a current flowing down your arm touching the topload. The return path is simply to ground via the ion wind I reckon.

Pinky's Brain

Thu Jun 28 2012, 05:51PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

The corona current isn't necessary to make your hair stand up though, the initial charge flowing into you is enough for that.

PS. the gravity working on your hair is a source of capacitance from the VDG's point of view.

Steve Conner

Thu Jun 28 2012, 06:07PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Indeed, any time electricity is doing work, like raising your hair, current is flowing and it is an electrodynamic problem.

The rearrangement of electrons in the rod is an ordinary conduction current. Displacement current "flows" between the sphere and the rod.

Ash Small

Thu Jun 28 2012, 06:45PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Ash Small wrote ...

I'm working on a test setup using my sig.gen. I have to go out later, but I'll post results as soon as I have them. I'm not sure if the sig. gen. will provide sufficient output without amplification, but I'll try this with and without the additional cap. that you suggest.

Initial test just using the sig. gen. and some small capacitors I had lying around (sig. gen. output ~12V, do not produce a measurable voltage (with my DMM) using my original circuit above, but with Steve's suggested additional cap. from one 'top node' to 'ground' (other terminal of sig. gen.) produces 2.2V accross the top capacitor. (I also had a 150 Ohm resistor accross the sig. gen outputs for some 'protection'.

I want to retry this using an AC flyback and large HV capacitors, along with suitable diodes though. In the meantime I may play around with it a bit more.

EDIT: forgot to say, frequency was 1MHz.

EDIT EDIT: When I connect my 'scope accross the top capacitor WITHOUT STEVE'S PROPOSED EXTRA CAPACITOR I'm getting a signal of ~100mV. I'm not sure yet if this means current is flowing WITHOUT a return path, or if it's just interference. I definitely seem to be getting current flowing accross the lower 'transmitting/recieving ' capacitor though.

I need to do a bit more experimental research here.

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